Semiconductor light emitting device and method for manufacturing the same

ABSTRACT

Certain embodiments provide a semiconductor light emitting device including: a first metal layer; a stack film including a p-type nitride semiconductor layer, an active layer, and an n-type nitride semiconductor layer; an n-electrode; a second metal layer; and a protection film protecting an outer circumferential region of the upper face of the n-type nitride semiconductor layer, side faces of the stack film, a region of an upper face of the second metal layer other than a region in contact with the p-type nitride semiconductor layer, and a region of an upper face of the first metal layer other than a region in contact with the second metal layer. Concavities and convexities are formed in a region of the upper face of the n-type nitride semiconductor layer, the region being outside the region in which the n-electrode is provided and being outside the regions covered with the protection film.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. Ser. No. 12/874,475, filed Sep. 2, 2010now U.S. Pat. No. 8,178,891, and claims the benefit of priority fromprior Japanese Patent Application No. 2010-46905 filed on Mar. 3, 2010in Japan, the entire contents of which are incorporated herein byreference.

FIELD

Embodiments described herein relate to a semiconductor light emittingdevice and a method for manufacturing the semiconductor light emittingdevice.

BACKGROUND

To achieve high efficiencies and high outputs, nitride semiconductorlight emitting devices (hereinafter also referred to as LEDs (LightEmitting Diodes)) designed for white lighting devices are being improvedin crystalline structures and device structures, and higher internalquantum efficiencies and higher light extraction efficiencies are beingrealized.

Where a nitride semiconductor is crystal-grown, a sapphire substrate isoften used, because it is inexpensive, and stable in high temperature. Acrystal growth with high crystallinity can be performed on a sapphiresubstrate with a low-temperature buffer. However, being an insulator, asapphire substrate does not have conductive properties and is low inthermal conductivity. Therefore, electrodes cannot be formed on the backface side of a sapphire substrate, and p- and n-electrodes need to beformed on the nitride semiconductor side. Therefore, the tendency tocause higher series resistance and the low heat release propertiesduring a high-power operation become problems in achieving even higherefficiencies and outputs.

A thin-film nitride semiconductor LED is known as one of the LEDstructures that eliminate the above problems and improve luminousefficiencies and outputs. Such a thin-film nitride semiconductor LEDtransfers LED structural crystals grown on a sapphire substrate ontoanother supporting substrate such as a Si substrate, a copper substrate,or a gold substrate. As devices are formed after the transfer onto asupporting substrate having conductive properties and high thermalconductivity, the current spread becomes larger by verticalenergization, and the electric conductive properties are improved.Further, the heat release properties are also improved. Also, by forminga structure that has an n-layer as an upper face through a transfer andextracts light from the n-layer side, a transparent electrode fordiffusing current becomes unnecessary for the n-layer having lowerresistance than a p-layer. Since light is not absorbed by a transparentelectrode, the light extraction efficiency becomes higher.

This process of transfer used here includes a process to bond crystals(epitaxial crystals) formed through an epitaxial growth to thesupporting substrate, and a lift-off process to detach the epitaxialcrystals from the sapphire substrate. The bonding process may involve aplating technique or a joining technique utilizing weight and heat, andthe lift-off process may involve a laser lift-off technique utilizingthermolysis of an interface caused by a laser or a chemical lift-offtechnique.

In such a thin-film LED structure, the difference in refractive indexbetween the surface of a GaN substrate and the external air is as largeas 2.5 times where only a laser lift-off process has been carried out,and the light reflection from the boundary face lowers the lightextraction efficiency.

To counter this problem, a technique of producing concavities andconvexities on the surface of each chip has been suggested. Theconcavities and convexities are formed by regrowing, polishing, andetching an n-type nitride semiconductor layer. According to a method forsimple formation, concavities and convexities are formed by rougheningthe surface through alkaline etching performed on the n-layer on theupper face of a GaN substrate on a supporting substrate. In this manner,the light extraction efficiency is made higher. To sufficiently increasethe light extraction efficiency, it is necessary to subject each devicecontaining epitaxial crystals to processing in an alkaline solution fora sufficiently long period of time. Therefore, formation of a protectionfilm that protects the epitaxial crystals and prevents short-circuitingand leakage is critical.

However, if a conventional protection film is subjected to long-timeprocessing with a high-density alkaline solution, not only the surfaceof the subject n-layer but also the side faces of the epitaxial crystalsand the active layer are etched, resulting in luminous efficiencydegradation, leakage, and short-circuiting. Also, cracks might be formedin the protection film and the epitaxial crystals due to the load andthe variation in temperature during the joining process and the largeimpact of the gas pressure or the like caused by the laser lift-offprocess performed when thin-film LEDs are formed. Therefore, thechallenge is to obtain a highly-reliable semiconductor light emittingdevice that has such a rough surface as to achieve higher lightextraction efficiency in the surface of a nitride semiconductor, and hasfewer defects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) through 1(c) are cross-sectional views showing proceduresfor manufacturing semiconductor light emitting devices according to anembodiment;

FIGS. 2( a) and 2(b) are cross-sectional views showing procedures formanufacturing semiconductor light emitting devices according to theembodiment;

FIGS. 3( a) through 3(c) are cross-sectional views showing proceduresfor manufacturing semiconductor light emitting devices according to theembodiment;

FIG. 4 is an electron micrograph of the surface having concavities andconvexities formed thereon in a semiconductor light emitting deviceaccording to the embodiment;

FIGS. 5( a) and 5(b) are electron micrographs of cross sections ofsemiconductor light emitting devices according to the embodiment and acomparative example;

FIG. 6 is a plan view of the supporting substrate prior to the divisioninto respective devices;

FIG. 7 is a cross-sectional view of a semiconductor light emittingdevice according to the embodiment; and

FIG. 8 is a graph showing the light extraction efficiencies ofsemiconductor light emitting devices according to the embodiment and thecomparative example.

DETAILED DESCRIPTION

Certain embodiments provide a semiconductor light emitting deviceincluding: a substrate having a first face and a second face opposed tothe first face; a first metal layer having a lower face facing to thefirst face of the substrate and an upper face; a stack film including ap-type nitride semiconductor layer having a lower face facing to theupper face of the first metal layer and an upper face, an active layerprovided on the upper face of the p-type nitride semiconductor layer andincluding a multiquantum well structure of a nitride semiconductor, andan n-type nitride semiconductor layer having a lower face facing to theactive layer and an upper face, the stack film having a tapered shape incross-section, with an area of a film plane gradually increasing fromthe n-type nitride semiconductor layer toward the p-type nitridesemiconductor layer; an n-electrode provided in a partial region of theupper face of the n-type nitride semiconductor layer; a p-electrodeprovided on the second face of the substrate; a contact electrodeprovided in a partial region of the lower face of the p-type nitridesemiconductor layer; a second metal layer having a lower face facing tothe upper face of the first metal layer and an upper face facing to thelower face of the p-type nitride semiconductor layer, the second metallayer covering the contact electrode and being in contact with thecontact electrode and the first metal layer, and the second metal layerhaving a minimum diameter that is smaller than a minimum diameter of theupper face of the first metal layer but is larger than a minimumdiameter of the lower face of the p-type nitride semiconductor layer;and a protection film protecting an outer circumferential region of theupper face of the n-type nitride semiconductor layer, side faces of thestack film, a region of the upper face of the second metal layer otherthan a region in contact with the p-type nitride semiconductor layer,and a region of the upper face of the first metal layer other than aregion in contact with the second metal layer, wherein concavities andconvexities are formed in a region of the upper face of the n-typenitride semiconductor layer, the region being outside the region inwhich the n-electrode is provided and being outside the regions coveredwith the protection film.

The following is a detailed description of an embodiment, with referenceto the accompanying drawings.

Referring to FIGS. 1( a) through 4, semiconductor light emitting devicesaccording to the embodiment are described. FIGS. 1( a) through 3(c) showthe procedures for manufacturing semiconductor light emitting devicesaccording to the first embodiment.

First, nitride semiconductor layers are sequentially grown on asubstrate (a wafer) for growing nitride semiconductor crystals or asapphire substrate 10 by metal organic chemical vapor deposition(MOCVD), for example. More specifically, a GaN layer 12 to be a bufferlayer, an n-type GaN layer 14, an active layer 16 of a multiquantum wellstructure made of InGaN, and a p-type GaN layer 18 are sequentiallygrown in this order on the sapphire substrate 10 (FIGS. 1( a)).

P-electrodes (reflecting contact electrodes) 20 are then formed withstack films of Ni and Ag on the p-type GaN layer 18 (FIG. 1( b)). Thep-electrodes 20 are formed for respective semiconductor light emittingdevices. An adhesive metal film 22 having Ti, Pt, and Au films that areto serve as adhesive metals and are stacked in this order is formed overthe nitride semiconductor crystal films 12, 14, 16, and 18, to cover thep-electrodes 20 (FIG. 1( b)). With this arrangement, the portions of theadhesive metal film 22 in the regions where the p-electrodes 20 areformed are turned into convex portions, and the portions of the adhesivemetal film 22 in the regions where the p-electrodes 20 are not formedare turned into concave portions (FIG. 1( b)). Patterning is thenperformed on the adhesive metal film 22 by a known lithographytechnique. After that, patterning is further performed on the stack film(the nitride semiconductor crystal films) including the p-type GaN layer18, the active layer 16, the n-type GaN layer 14, and the GaN layer 12(FIG. 1( c)).

Through the patterning, the nitride semiconductor crystal films on thewafer are turned into a mesa having a tapered shape in cross-section,with the area of the film plane gradually increasing from the area ofthe film plane of the p-type GaN layer 18 to that of the GaN layer 12.Here, the “film plane” means the upper plane of each of the layers. Whenpatterning is performed on the stack film, a patterned adhesive metalfilm may be used as a mask. Alternatively, patterning may be performedon the stack film before the adhesive metal film 22 is formed, and afterthe patterning, the adhesive metal film 22 may be formed.

Meanwhile, an Au—Sn layer 32 to be an adhesive metal film is formed on aSi substrate 30 to be a supporting substrate (FIG. 2( a)). The adhesivemetal film 22 on the sapphire substrate 10 and the adhesive metal film32 on the Si substrate 30 are placed to face each other, and pressure isapplied to them at a high temperature of 250° C. or higher over acertain period of time, so that the adhesive metal film 22 on thesapphire substrate 10 and the adhesive metal film 32 on the Si substrate30 are bonded to each other. In this bonding, the contact electrodes 20are buried into the adhesive metal film 32, since the melting-pointtemperature of the contact electrodes 20 is much higher than themelting-point temperature of the adhesive metal film 32 (FIG. 2( a)).

As shown in FIG. 2( b), pulse irradiation is then performed with a UV(Ultra-Violet) laser or a KrF laser of 248 nm in wavelength from theside of the sapphire substrate 10, for example, so as to detach thesapphire substrate 10 from the nitride semiconductor crystal films 12,14, 16, and 18. The surface of the GaN layer 12 exposed at this point isthe surface to be subjected to wet etching.

Patterning is then performed on the nitride semiconductor crystal films12, 14, 16, and 18 by a known lithography technique, to divide thenitride semiconductor crystal films 12, 14, 16, and 18 intosemiconductor light emitting devices. At this point, patterning is notperformed on the adhesive metal film 22, and the adhesive metal film 22is left exposed among the nitride semiconductor crystal films dividedinto the semiconductor light emitting devices. The patterned nitridesemiconductor crystal films are turned into mesas each having a taperedshape in cross-section, with the area of the film plane graduallyincreasing from the area of the film plane of each GaN layer 12 to thatof each p-type GaN layer 18 (FIG. 3( a)).

A SiO₂ film 40 as a protection film is then formed to cover the surfacesof the nitride semiconductor crystal films of tapered shapes and theexposed adhesive metal films 22 and 32, for example (FIG. 3( b)). Thenitride semiconductor crystal films form mesa structures, the minimumdiameter of each lower face of the nitride semiconductor crystal filmsin contact with the adhesive metal film 22 is smaller than the minimumdiameter of the upper face of the adhesive metal film 22, and theminimum diameter of the lower face of the adhesive metal film 22 incontact with the adhesive metal film 32 is smaller than the minimumdiameter of the upper face of the adhesive metal film 32. In otherwords, the structure has a folding-fan shape. Accordingly, the adhesivemetal film 22 is in tight contact with the peripheral end region of eachlower portion of the nitride semiconductor crystal films each having amesa shape, and the protection layer 40 without a step separation can beformed, without a void formed between the protection layer 40 and theadhesion metal films 22 and 32.

The protection layer 40 covering the upper face of each semiconductorlight emitting device is then removed. However, the protection layer 40remains on the outer circumferential region of the upper face of eachsemiconductor light emitting device (the upper face of each GaN layer12). With this arrangement, the upper face of each semiconductor lightemitting device is exposed, except for the outer circumferential regionof each upper face (FIG. 3( c)). N-electrodes 44 are then formed at thecenter portions of the exposed upper faces of the GaN layers 12 (FIG. 3(c)). As the material of the n-electrodes 44, it is preferable to use analkali-resistant electrode material. It is particularly preferable touse a material containing one of the following metals: Pt, Au, Ni, andTi. By using such a material, the sizes (height differences) of theconcavities and convexities formed in the upper faces of the GaN layers12 by the later described alkaline etching can be made larger.

After that, etching is performed on the exposed upper faces of the GaNlayers 12 with the use of an alkali solution, to roughen the exposedupper faces of the GaN layers 12. In this manner, the GaN layers 12 areturned into GaN layers 12 a each having concavities and convexitiesformed in its exposed upper face (FIG. 3( c)). This is supposedlybecause electrons or holes travel between the surfaces of the GaN layers12 and the n-electrodes 44 at the time of etching, and anelectrochemical reaction is caused in each surface, accelerating theetching. In this embodiment, a potassium hydroxide solution of 1 mol/lin density and 70° C. in temperature is used as the alkaline solution,and etching is performed for 15 minutes. As the etching smoothlyprogresses, the surface becomes clouded. While being immersed in thepotassium hydroxide solution, the concavities and convexities areexposed to UV rays, and can be made even larger as a result. Theconcavities and convexities can also be made larger by performingetching while a voltage of is intermittently applied between then-electrodes 44 and the GaN layers 12. The sizes of the concavities andconvexities are several hundreds of nanometers to several micron meters.

FIG. 4 shows an electron micrograph of the upper face of a GaN layer 12having concavities and convexities formed in the above described manner.As can be seen from FIG. 4, the concavities and convexities vary insize. Accordingly, the reflection from the boundary surface between eachGaN layer 12 and the air becomes smaller, and the light extractionefficiency can be made higher.

In this embodiment, the minimum diameter of the lower face in contactwith the adhesive metal film 22 is smaller than the minimum diameter ofthe upper face of the adhesive metal film 22, and the minimum diameterof the lower face of the adhesive metal film 22 in contact with theadhesive metal film 32 is smaller than the minimum diameter of the upperface of the adhesive metal film 32. Because of this folding-fan shape,part of the upper face and the side faces of the nitride semiconductorcrystal films, and the joined portions between the side faces and theadhesive metal are covered with the protection layer 40 without a stepseparation. Accordingly, even if the upper face of the nitridesemiconductor crystal films is roughened with the use of an alkalinesolution, the active layer and the reflecting contact electrodes 20 canbe thoroughly protected. Thus, higher reliability and higher lightextraction efficiency are achieved. Also, reflecting contact electrodes20 each having a large area can be formed, and the reflectivity can bemade higher. Furthermore, a decrease in operating voltage can beexpected, since large reflecting contact electrodes can be formed. Also,since the protection layer 40 without a step separation is formed,leakage and short-circuiting in devices due to metal adherence or thelike during the manufacturing procedures can be prevented. Further,since the protection layer 40 without a step separation is formed, theprocess to manufacture thin-film semiconductor light emitting devicescan be tolerated, even though the process involves intensified impactsfrom the transfer, the bonding, and the laser lift-off technique, forexample. Also, cracks and the likes are not formed in the protectionlayer 40.

As a comparative example of this embodiment, a semiconductor lightemitting device is formed. This semiconductor light emitting device ismanufactured in the same manner as in this embodiment, except that thearea of the lower face of the nitride semiconductor crystal films incontact with the adhesive metal film 22 is the same as the area of theupper face of the adhesive metal film 22. FIGS. 5( a) and 5(b) showelectron micrographs of sections of this embodiment and the comparativeexample, respectively. As can be seen from FIGS. 5( a) and 5(b), no stepseparations are formed in the protection layer in this embodiment, but astep separation is formed in the peripheral end region of the lowerportion of the nitride semiconductor crystal films in a mesa shape inthe comparative example. If a step separation is formed in a protectionlayer, cracks and the likes are easily formed in the protection layerduring the manufacturing process. Also, if a protection layer having astep separation is formed as in the comparative example, the alkalineetching solution enters the device through the step separation andcracks, and the active layer and the reflecting contact electrode areetched. As a result, current leakage occurs, and outputs are degraded.

FIG. 6 is a plan view of semiconductor light emitting devices seen fromthe side of the n-electrodes 44 after the concavities and convexitiesare formed. As can be seen from FIG. 6, undivided devices are placed onthe Si substrate 30. After that, a p-electrode 46 is formed on the faceof the silicon substrate 30 on the opposite side from the side on whichthe n-electrodes 44 are formed, as shown in FIG. 3( c).

After the procedures shown in FIG. 3( c) are completed, dicing isperformed on the protection layer 40, the adhesive metal films 22 and32, and the Si substrate 30, to divide them into respectivesemiconductor light emitting devices. In this manner, the semiconductorlight emitting device shown in FIG. 7 is completed. The semiconductorlight emitting devices of the above described comparative example arealso divided by dicing. The light extraction efficiency of eachsemiconductor light emitting device of this embodiment manufactured inthe above described manner is 1.5 times higher than the light extractionefficiency of each semiconductor light emitting device of thecomparative example, and is 1.3 times higher than the light extractionefficiency observed in a case where alkaline etching is not performed,as shown in FIG. 8.

As described so far, this embodiment can provide semiconductor lightemitting devices having high light extraction efficiency, and a methodfor manufacturing such semiconductor light emitting devices.

The supporting substrate may be a silicon substrate, a silicon carbidesubstrate, a substrate formed by bonding germanium to a siliconsubstrate, or a substrate formed by plating a silicon substrate with ametal such as copper. The silicon substrate may be a substrate that hasa plane orientation of (111), (110), or (100), and also has an offangle.

As for the protection layer, it is preferable to use a material thatcontains silicon dioxide, silicon nitride, zirconium oxide, niobiumoxide, or aluminum oxide.

The chemical solution used in the alkaline etching may betetramethylammonium hydroxide, other than potassium hydroxide.

As the reflecting contact electrodes, it is desirable to use aluminum,other than silver.

The adhesive metal film 22 preferably contains titanium, platinum, gold,or tungsten.

As the adhesive metal film 32, it is possible to use a low-melting-pointmetal that is a metal eutectic such as Au—Si, Ag—Sn—Cu, or Sn—Bi, or anon-solder material such as Au, Sn, or Cu, other than Au—Sn.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the sprit ofthe inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and sprit of the invention.

What is claimed is:
 1. A semiconductor light emitting device comprising: a substrate; at least one metal layer provided on the substrate; a stack film including a p-type nitride semiconductor layer provided on the at least one metal layer, an active layer provided on the p-type nitride semiconductor layer and including a multiquantum well structure of a nitride semiconductor, and an n-type nitride semiconductor layer provided on the active layer; an n-electrode connected to the n-type nitride semiconductor layer; a p-electrode connected to the p-type nitride semiconductor layer; and a protection film protecting an outer circumferential region of an upper face of the n-type nitride semiconductor layer, side faces of the stack film, and a region of an upper face of the at least one metal layer other than a region in contact with the p-type nitride semiconductor layer, wherein the stack film has a tapered shape in cross-section, with an area of a film plane gradually increasing from the n-type nitride semiconductor layer toward the p-type nitride semiconductor layer, and concavities and convexities are formed in a region of the upper face of the n-type nitride semiconductor layer, and wherein the at least one metal layer comprises: a first metal layer provided between the substrate and the p-type nitride semiconductor layer; and a second metal layer provided between the substrate and the first metal layer, and the second metal layer having a minimum diameter that is smaller than a minimum diameter of the first metal layer but is larger than a minimum diameter of the p-type nitride semiconductor layer.
 2. The device according to claim 1, further comprising a contact electrode provided between the p-type nitride semiconductor layer and the at least one metal layer.
 3. The device according to claim 2, wherein the contact electrode contains one of silver and aluminum.
 4. The device according to claim 1, wherein the substrate is one of a silicon substrate, a silicon carbide substrate, a copper substrate and a substrate formed by bonding germanium to a silicon substrate.
 5. The device according to claim 1, wherein the second metal layer contains one of titanium, platinum, gold, and tungsten.
 6. The device according to claim 1, wherein the protection film is one of a silicon dioxide layer, a silicon nitride layer, a zirconium oxide layer, a niobium oxide layer, and an aluminum oxide layer.
 7. The device according to claim 1, wherein the first metal layer is one of an Au—Sn layer, an Au—Si layer, an Ag—Sn—Cu layer, an Sn—Bi layer, an Au layer, an Sn layer, and a Cu layer. 